Further to my previous
You did not specify what article your question was referencing. I went from memory and knew that in my talk on the Electron
I had considered the problem.
A few minutes later, while sitting on the couch with the two dogs, re-watching the final round of The Open, I realized you might be referencing Quantum World
where I say:
Mass and energy are not "united into a single concept".
Mass can not be changed to energy.
Energy can not be changed to mass.
The point here is that the "mass" and "energy" being talked about are two different things. Later I use the analogy of a drop of oil tainting a million gallons of water -- this is about all it takes before we can taste it, by the way. The point is that the oil is not the water, and -- when/if the water could be "instantly purified" -- the oil would not have become water.
Mass ties up springs. Damps them down. When you remove a mass, the springs -- all being at the same level of energy -- merely revert to their high energy vibration. They are simply no longer hampered.
A key unlocks a door, and behind that door could be a wall of water. But the key did not become the water.
We are so used to thinking of burning something, and getting linear amounts of energy from linear amounts burned. But what is really happening is energy adjustment; energy optimization. The wood or coal or hydrocarbon were all forms of energy before and the resultant gases and byproducts are all forms of energy after.
Molecules of, say a hydrocarbon were at one energy level, and after combusion the resultant molecules of carbon dioxide and water are at a net lower level. Exothermic reactions are downhill. The point is that gasoline didn't _become_ CO2 and H2O -- that is not a low-level way to think of it. Instead, atoms of C, O and H found a lower collective energy state, and went willingly downhill to get there. And, by the way, that new lower level state is lower in energy because less of the ultra high energy quanta -- I call them springs -- are tied up in it. The "energy" it gave off was the liberation of the higher energy quanta that permeate the universe. An exothermic reaction is a reaction that liberates ultra high energy units. An endothermic reaction is a reaction that ties up more ultra high energy units in the end than were tied up in the starting materials.
In thinking still further about this second place where I talked about mass-energy -- Quantum World -- I would stay I stand by the first and third sentences. One: Mass and energy are not "united into a single concept" -- at least not by relativity nor quantum mechanics. Three: Energy can not be changed to mass. As to Two: Mass can not be changed to energy -- this is probably not quite right. The mass that fuses does become that ultra high energy. But this does not produce much in itself. Rather, it is the liberation of the ultra high energy springs -- no longer tied up with the mass -- that are the major energy release.